It is known in the prior art to separate oxygen from air using a molten stream of an oxygen acceptor comprising a liquid containing alkali metal nitrate and nitrate salts. This fundamental chemical air separation is set forth in U.S. Pat. No. 4,132,766. At least some energy of compression of the air feed to such a separatory process is recovered by the expansion of oxygen depleted gas from the separatory process.
The coproduction of oxygen and nitrogen as relatively pure products of a chemical separation utilizing the alkali metal salts of nitrite and nitrate materials is also known. In U.S. Pat. No. 4,287,170, air is contacted sequentially with such alkali metal salts, and then residual oxygen is scavenged from the oxygen depleted effluent with an absorption media, such as manganese oxide. At least some energy of compression for the feed air is recovered by expanding the nitrogen product to a lower pressure.
This prior art (U.S. Pat. No. 4,132,766 and U.S Pat. No. 4,287,170) is not uniquely integrated by heat exchange with a combustion process to co-produce a high temperature process stream, oxygen and nitrogen.
U.S. Pat. No. 4,340,578 discloses a method for producing oxygen with a chemical absorbent solution of molten alkali metal nitrite and nitrate salts wherein the salt solution contains additional oxides in low concentration, and the oxygen depleted effluent from the chemical separation is combusted with fuel and expanded to recover power in two stages. The combustion effluent is heat exchanged with the air feed and the oxygen product to elevate the air feed to absorption conditions. The molten salt absorbent solution is depressurized to release the reversibly contained oxygen therefrom and provide an oxygen product.
Other patents of interest to the use of alkali metal nitrate and nitrite molten salt absorbents for oxygen include: U.S. Pat. Nos. 4,521,398, 4,526,775, 4,529,577, 4,708,860 and 4,800,070.
U.S. Pat. No. 3,310,381 discloses the recovery of oxygen from air using a suspension of solid absorbent in a liquid carrier in a cocurrent contact of air and absorbent. Temperatures above 500.degree. C. are recited for the system which uses barium oxide and barium peroxide. The patent process is a continuous version of the Brin process using a pressure and temperature swing cycle. Feed air cocurrently contacts the barium oxide/barium peroxide suspension in an absorber which heat exchanges with an external heat exchange fluid. The absorber operates at approximately 600.degree. C. and a pressure slightly above atmospheric pressure. The oxidized acceptor is further heated to approximately 800.degree. C. in a heater. The high temperature oxidized acceptor is reduced in pressure and desorbs oxygen with attendant reduction in temperature to 720.degree. C. The partial pressure of the oxygen in the acceptor is determined by temperature because the barium oxide and barium peroxide are always present in the suspension of acceptor.
U.S. Pat. No. 4,617,182 is directed to a chemical separation of oxygen from air using the high temperature heat from a stand-alone process to effect the desorption of oxygen from a chemical absorbent. In addition, the patent discloses the use of a salt-to-salt heat exchanger which effectively transfers heat to a low temperature chemical absorbent from a high temperature chemical absorbent in the same cyclic loop.
U.S. Pat. No. 4,746,502 discloses a similar air separation system to recover oxygen using a chemical absorbent wherein a salt-to-salt heat exchanger is used to indirectly transfer heat from the high temperature chemical absorbent to low temperature chemical absorbent, all within different portions of a cyclic, closed system.
The evolution of oxygen and the simultaneous formation of a solid phase during the cooling of a melt composed of V.sub.2 O.sub.5 and a metal oxy salt was observed as early as 1880 by P. Hautefeville, C. R. Acad. Sci., 90, 744 (1880). These reactions were examined further during the early 1990's by W. Prandtl and H. Murschhauser as reported in Z. Anorg. Allg. Chem., 60, 441 (1908) and G. Canneri, Gazzetta Chimica Italiana, 58, 6 (1928). It was found that when the oxy salt was Na.sub.2 O or K.sub.2 O, the maximum quantity of oxygen was obtained when the mole ratio of metal oxy salt to V.sub.2 O.sub.5 was approximately 0.15 to 1.
It was not until the last three decades that these solid phases, termed vanadium bronzes, have been extensively studied and characterized. A review by E. Banks and A. Wolf gives a good count of the work done on the vanadium bronzes, as well as other transition metal bronzes, up to 1968, as reported in E. Banks and A. Wold, in "Preparative Inorganic Synthesis," vol. 4, W. L. Jolly, editor, Interscience, New York, 1968, pp. 237 through 268. The vanadium bronzes are still of interest, particularly in terms of their physical and catalytic properties. A recent article references several catalytic papers and describes an optical microscopic study of the oxygen evolution during the formation of the .beta.K.sub.0.23 V.sub.2 O.sub.5 bronze as reported in J. Vandenberg, H. A. Van Dillen, J. W. Gens, and M. C. Stolk, Ber. Bunsenges. Phys. Chem., 87, pp. 120 through 123 (1983).
Using the known vanadium bronzes in a cyclic continuous process, such as is exemplified by the air separation techniques described in the patents above using alkaline metal salts of nitrate and nitrite, the present invention provides an effective and energy efficient method of recovering oxygen as well as nitrogen using the high temperature heat from preferably stand-alone industrial processes wherein the heat is utilized in a novel context to provide such oxygen and nitrogen and residual usable heat for a bottoming cycle as will be described in greater detail below.